Wim De Waele

Optical measurement of target displacement and velocity in bird strike simulation experiments

Thousands of bird strikes on aircraft occur annually with subsequent consequences for repair costs but also for public safety. In bird strike simulation experiments a bird or birdlike object is usually accelerated towards a target. Knowledge of the parameters of the impact, such as the velocities involved, the target displacement and the energy transferred from the projectile to the target, lead to a better understanding of the phenomenon, and eventually to better protection against damage caused by bird strikes. However, due to the extremely high velocities and energies involved in the experiments, methods for registration of the impact parameters are not obvious. Non-contact measurement techniques have a number of advantages over the more common mechanical contact methods. In this paper, a relatively simple optical technique is presented for recording the history of target displacement, from which the target velocity and energy can be readily obtained. The technique is based on the relative displacement of two moire line gratings: one grating attached to the target and the other serving as a stationary reference grating. The proposed technique has proved to be useful. Results of a representative bird strike experiment are presented.

Investigation of strain measurements in (curved) wide plate specimens using digital image correlation and finite element analysis

Some pipelines face global plastic straining due to the nature of their installation process or harsh environmental conditions during operation. The ability of the girth welds to withstand these plastic strains is often evaluated on the basis of wide plate tests. Key for the validity of these tests is a representative measurement of remote strain, mostly obtained by linear variable differential transformers and/or strain gauges. The outcome of the remote strain measurement depends on the specimen geometry and the position of these sensors. In an attempt to investigate a specific geometric design of wide plate specimens and to find appropriate remote strain sensor positions, the authors have performed a series of tension tests on medium-sized wide plate specimens, supported by digital image correlation strain measurements. In addition, finite element simulations have been performed to evaluate whether the experimental observations can be extrapolated to a wider range of conditions. The results indicate that the strain distribution is mostly influenced by the weld strength mismatch, which governs the lateral restraint. For all experiments and simulations, nevertheless, the strain field was highly uniform in an identified zone, resulting in simple guidelines regarding specimen geometry and sensor positioning.

Parametric finite element model for large scale tension tests on flawed pipeline girth welds

The structural response of a pipe with a flawed girth weld, subjected to global plastic deformation is influenced by a large number of geometrical and material properties. Finite element models that aim to simulate this need to include all relevant influence factors, which causes high challenges in the creation of such models. In search for a high degree of flexibility, automation and ease of use, the authors have developed a parametric script that creates geometries for two common pipeline girth weld tension tests: the curved wide plate test and the full scale pressurized pipe tension test. The developed model allows to modify pipe geometry, test specimen geometry, flaw position (weld metal centre or heat-affected zone), flaw size, weld misalignment, pipe thickness variations, weld fusion line profile, and weld cap profile. The desired geometry is obtained by a coordinate transformation scheme that starts from a flat plate with a simplified weld geometry. A deliberate partitioning strategy is applied to obtain flexibility in the flaw location and full independence between a fine flaw mesh and a coarse body mesh. This article describes the approach, structure and governing equations of the model. An example geometry is discussed to illustrate the various possibilities. The proposed model provides inspiration for all who seek to develop parametric finite element models with a similar flexibility and ease of use.

Nonlinear Contact Analysis of Different API Line Pipe Coupling Modifications

Threaded pipe couplings are used to join pipelines when they have to be uncoupled frequently or as an easy to assemble alternative to welding. A large variety of patented coupling modifications are available, but little is known about their influence on the connection s behavior. In this study, the finite element model of an API line pipe threaded pipe connection is presented and its nonlinearities in material properties and contact behavior are discussed. Test results obtained from a four-point bending fatigue experiment are in good agreement with the results of the numerical simulations. A series of modifications of the standard connection are simulated to gain a better understanding in the influence of geometrical and material parameters on the connections performance. It was found that not all existing coupling modifications are improving the connections performance. It can be concluded that critical evaluation of the performance of existing coupling modifications is necessary and finite element analyses are proven to be a useful tool for this.

Pressure Correction Factor for Strain Capacity Predictions Based on Curved Wide Plate Testing

Strain-based girth weld defect assessment procedures are essentially based on large scale testing. Ever since the 1980s curved wide plate testing has been widely applied to determine the tensile strain capacity of flawed girth welds. However, the effect of internal pressure is not captured in curved wide plate testing. Accordingly, unconservative predictions of strain capacity occur when straightforwardly transferred to pressurized pipes. To address this anomaly, this paper presents results of finite element simulations incorporating ductile crack growth. Simulations on homogeneous and girth welded specimens indicate that a correction factor of 0.5 allows to conservatively predict the strain capacity of a pressurized pipe through wide plate testing under the considered conditions.

Strain monitoring of FRP elements using an embedded fibre optic sensor

This paper presents a strain monitoring approach for following up FRP elements (in this case a [90°] CFRP laminate) using an embedded fibre optic sensor. The sensor exists of two fibre Bragg gratings (FBGs) written in a polarization maintaining fibre (PMF). First, the strain response of the non-embedded sensor is determined which makes it possible to relate the different bragg peak shifts with the induced strain field in the core of the optical fibre. Secondly, a transfer coefficient matrix is presented and calculated using finite element simulations which relates the measured strain field of the sensor with the adjacent one existing in the structure as if no sensor would be present.

Validating numerically predicted make-up of threaded connections using digital image correlation and infrared monitoring

To ensure a reliable connection between two pipe sections, an initial make-up is applied to the threaded connections to induce a favorable stress state. Using finite element analysis techniques, it is possible to predict the internal strains and stresses of the connection when torque is applied. This article presents the outline of an experimental setup, which allows to directly validate the occurring strains together with the torque versus turn diagram and indirectly the contact pressures. The strains are measured by means of digital image correlation and strain gages. Both methods provide similar results and comply with the predicted finite element analysis strains when taper mismatch is taken into account. In an effort to qualitatively validate the simulated contact pressures, the temperature of the box is measured during make-up by means of infrared monitoring. The maximum temperature increase occurs near the vanishing threads where contact pressures are larger. Despite promising results, no decisive validation for the contact pressures could be obtained.

Resonant Bending Fatigue Test Setup for Pipes With Optical Displacement Measuring System

Pipes and tubular members are used in offshore applications as structural elements, such as columns or in transport pipelines, risers, etc. When subjected to dynamic loads, weld defects or geometrical stress raisers can initiate fatigue cracks, causing the columns or pipelines to fail prematurely. In order to investigate the fatigue behavior of pipe joints, a resonant bending fatigue setup was designed, suitable for testing pipes within a diameter range from 6 in. to 20 in. In this setup, the pipe, filled with water, is subjected to a dynamic excitation force with a frequency close to the natural frequency of the filled pipe. The force is applied using a unique drive unit with excentric masses. The pipe is supported in the nodes of its natural wave-form, so that no dynamic forces are transmitted to the setup. The deformation of the pipe is measured at discrete locations using an optical 3D dynamic measuring system. Through-thickness fatigue cracks can be detected by pressurizing the water in the pipe and applying a pressure gauge. In this paper, some unique aspects of the design of the resonant bending fatigue setup are discussed by presenting the results of a semianalytical model used for calculating the deformation and bending stress in the excitated pipe and by comparing these results to the deformation measurements made by the dynamic measuring system. The working principles of the setup are illustrated by showing the preliminary test results for a 12 in. diameter X65 steel pipe with a wall thickness of 12.7 mm. It is demonstrated that the model predicts the behavior of the pipe in the setup very accurately.

Finite Element Analysis of Influence of Material Anisotropy on the Springback of Advanced High Strength Steel

TRIP 870 advanced high strength steel has been chosen for a detailed investigation on the effects of mechanical anisotropy on the springback observed in sheet metal forming. A series of finite element simulations using the Hill’48 anisotropic yield criterion and a standard U‐shape forming test based on the NUMISHEET’93 benchmark have been performed. A detailed study of appropriate contact definitions and stress‐strain representation has been carried out. Finally, a comparison of springback angles for different orientations of sheet strips relative to the rolling direction is presented.

Effect of Pipe and Weld Metal Post-Yield Characteristics on Plastic Straining Capacity of Axially Loaded Pipelines

Girth welds in pipelines subject to longitudinal plastic tensile strains are critical regions of the pipeline. As girth welds might contain flaws of some form or other, it is of paramount interest to have a thorough understanding of the deformation characteristics of girth welds in the post-yield loading range. The response of a defective weld to plastic strains depends on many variables. While toughness is an important variable, large-scale tests demonstrate that the plastic straining capacity is directly affected by the mechanical properties of the materials surrounding the defect. The purpose of this paper is to describe the effect of the interrelation between the pipe and weld metal post-yield characteristics on the straining capacity of girth welds containing a defect.Copyright